As the user demands higher and higher operating speed of the computer, the power required by the central processing unit and the peripheral devices grows correspondingly, and thus, the power supply used in the computer system needs to provide a higher power output; however, considering the harm that the electricity can do on the human body, the power supply has an energy-limiting standard for its output power, i.e. maximum output power (Max VA), which is usually 240VA in the current specification; thus, if the output voltage is 12V, the maximum output current will be 20 A. For example, in SSI or UL, the electric device has an energy-hazard standard stipulating that maximum output power of a power supply is 240 VA in order to protect the computer user's safety.
With the fast-increasing operating speed of the computer system and more and more peripheral devices connected thereto, if persisting in high-level safety standard, one have to pay for the expense of additional power output ports. Each power output port needs a power-protection circuit, and the manufacturer has to fabricate various specifications of power supplies with different power-protection circuits demanded by different systems, which besets the manufacturer very much, needless to mention the difficulty of the design of the power-protection circuit. Furthermore, many high-level peripheral devices require a power higher than that regulated by a protection standard, and the user has to adopt the power supply without protection function; thus, to meet the market demand, the power supply's manufacturer has to provide the power supply with high power output but without protection function, which further raises the cost of design and fabrication and lacks economical profit.